1. Field of the Invention
This invention relates to medical devices. More particularly, this invention relates to cooling of tissue contacted by an invasive probe within the body.
2. Description of the Related Art
Cardiac arrhythmia, such as atrial fibrillation, occurs when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm. Important sources of undesired signals are located in the tissue region along the pulmonary veins of the left atrium and in myocardial tissue associated with cardiac ganglionic plexi. In this condition, after unwanted signals are generated in the pulmonary veins or conducted through the pulmonary veins from other sources, they are conducted into the left atrium where they can initiate or continue arrhythmia.
Procedures for treating arrhythmia include disrupting the areas causing the arrhythmia by ablation, as well as disrupting the conducting pathway for such signals. Ablation of body tissue using electrical energy is known in the art. The ablation is typically performed by applying alternating currents, for example radiofrequency energy, to the electrodes, at a sufficient power to destroy target tissue. Typically, the electrodes are mounted on the distal tip of an invasive probe or catheter, which is inserted into a subject. The distal tip may be tracked in a number of different ways known in the art, for example by measuring magnetic fields generated at the distal tip by coils external to the subject.
A known difficulty in the use of radiofrequency energy for cardiac tissue ablation is controlling local heating of tissue. There are tradeoffs between the desire to create a sufficiently large lesion to effectively ablate an abnormal tissue focus, or block an aberrant conduction pattern, and the undesirable effects of excessive local heating. If the radiofrequency device creates too small a lesion, then the medical procedure could be less effective, or could require too much time. On the other hand, if tissues are heated excessively then there could be local charring effects due to overheating. Such overheated areas can develop high impedance, and may form a functional barrier to the passage of heat. The use of slower heating provides better control of the ablation, but unduly prolongs the procedure.
Previous approaches to controlling local heating include the inclusion of thermocouples within the electrode and feedback control, signal modulation, local cooling of the catheter tip, and fluid-assisted techniques, for example perfusion of the target tissue during the energy application, using chilled fluids. Typical of the last approach is Mulier, et al. U.S. Pat. No. 5,807,395.
Commonly assigned U.S. Pat. No. 6,997,924, which is herein incorporated by reference, describes a technique of limiting heat generated during ablation by determining a measured temperature of the tissue and a measured power level of the transmitted energy, and controlling the power output level responsively to a function of the measured temperature and the measured power level.
More recently, commonly assigned U.S. Patent Application Publication No. 2010/0030209 by Govari et al., which is herein incorporated by reference, describes an insertion tube, having an outer surface with a plurality of perforations through the outer surface, which are typically about 100 μm in diameter, and are distributed circumferentially and longitudinally over the distal tip. A lumen passes through the insertion tube and is coupled to deliver a fluid to the tissue via the perforations.